Montumun kemeri gevşemiş, bu yüzden onu sıkılaştırıyorum.

Montumun breaks down as:

• mont = coat
• -um = my
• -un = genitive ending, roughly of

So montumun means of my coat.

This is because Turkish is using a possessive noun phrase:

• montumun kemeri = the belt of my coat / my coat’s belt

If you said montun, that would mean of the coat, not specifically of my coat.

In a Turkish possessive construction, the second noun usually takes a possessive ending.

So:

• kemer = belt
• kemeri = its belt / the belt (belonging to it)

That is why:

• montumun kemeri = my coat’s belt

This pattern is very common:

• evin kapısı = the house’s door
• arabanın rengi = the car’s color
• montumun kemeri = my coat’s belt

Gevşemiş comes from gevşemek, meaning to become loose or to loosen.

Here it means something like:

• has become loose
• has loosened
• is loose now

The ending -miş often gives a sense of:

• a result that is now noticeable
• something the speaker has realized or observed
• a past action with a present result

So Montumun kemeri gevşemiş suggests: My coat’s belt has come loose / My coat’s belt is apparently loose now.

Yes, gevşedi is possible, but it has a slightly different feel.

• gevşedi = it loosened / it became loose
  
  • more direct, plain past
• gevşemiş = it has become loose / it seems to have loosened
  
  • focuses more on the current result or discovery

In this sentence, gevşemiş sounds natural because the speaker notices the belt is loose and reacts by tightening it.

So:

• kemeri gevşedi = the belt loosened
• kemeri gevşemiş = the belt has loosened / seems to have come loose

Bu yüzden means:

• because of this
• for this reason
• therefore
• so

It connects the first idea to the result:

• Montumun kemeri gevşemiş, bu yüzden...
• My coat’s belt has loosened, so...

It is a very common way to show cause and result in Turkish.

Similar expressions include:

• bu nedenle = for this reason
• o yüzden = that’s why / so

Onu means it and refers to the belt (kemer / kemeri).

So the second clause means:

• onu sıkılaştırıyorum = I am tightening it

Turkish often avoids repeating the noun if it is already clear.

Instead of repeating kemeri, the sentence uses onu.

Because onu is the accusative form of o.

• o = he / she / it / that
• onu = him / her / it (as a direct object)

In onu sıkılaştırıyorum, it is the thing being tightened, so it is the direct object.

That is why Turkish uses onu, not o.

Sıkılaştırıyorum comes from:

• sıkı = tight
• sıkılaştırmak = to make tight, to tighten

So it very clearly means I am tightening it.

Sıkmak can also sometimes mean to tighten, but it has many other meanings too, such as:

• to squeeze
• to press
• to bore someone
• to fire a gun

Because of that, sıkılaştırmak is more precise here. It specifically means to make tighter.

It can be broken down like this:

• sıkı = tight
• -laştır- = make into / cause to become
• -ıyor = present continuous
• -um = I

So:

• sıkılaştırıyorum = I am making it tight = I am tightening it

This is a very useful pattern in Turkish:

• geniş = wide

• genişletmek = to widen

• dar = narrow

• daraltmak = to narrow

• sıkı = tight
• sıkılaştırmak = to tighten

Turkish often leaves out the subject pronoun when it is already clear from the verb ending.

Here, -um in sıkılaştırıyorum already tells you the subject is I.

So:

• (Ben) onu sıkılaştırıyorum
• I am tightening it

Both are possible, but ben is usually unnecessary unless you want emphasis.

A more literal order would be something like:

• My coat’s belt has loosened, because of this it I-am-tightening

Turkish usually places the verb at the end of the clause, and objects often come before the verb.

So:

• Montumun kemeri gevşemiş
• bu yüzden onu sıkılaştırıyorum

This is completely normal Turkish word order.

Yes. You could say:

• Montumun kemeri gevşemiş, bu yüzden kemeri sıkılaştırıyorum.

That is grammatically fine. But Turkish often prefers onu to avoid repeating the same noun too soon.

So:

• ... onu sıkılaştırıyorum = more natural in many contexts
• ... kemeri sıkılaştırıyorum = also possible, slightly more repetitive

Mont usually refers to a coat, outer jacket, or similar outerwear.

Depending on context, other words could be used:

• ceket = jacket
• palto = overcoat

So the structure stays the same:

• Ceketimin kemeri gevşemiş...
• Paltomun kemeri gevşemiş...

Only the clothing word changes.